1. Field of the Invention
The present invention relates to a control valve, such as a mass flow controller or a flow-rate controlling valve, for controlling a flow rate of various kinds of fluid such as a gas and a liquid, in which a valve is controlled by a thermal expansion force or an electromagnetic force.
2. Description of the Prior Art
One control valve of this type is disclosed in Japanese Patent Publication No. 59-2879. Its valve mouth is circular in shape and a valve head corresponding thereto is formed in a conical shape or a trapezoid conical shape so that said valve head may be partially put in said valve mouth to close the valve mouth.
Although it is desired that a difference between a primary pressure and a second pressure (hereinafter referred to as differential pressure) is in almost linear proportion to a flow rate under the condition that a pressure of a fluid is constant or said differential pressure is in almost linear proportion to a flow rate under the condition that either of the primary pressure and the secondary pressure is constant in a control valve of this type, a control valve constructed in the above described manner has shown a disadvantage in that if the differential pressure exceeds a certain definite value, the flow rate is decreased, as shown by a curve A in a differential pressure-flow rate characteristic diagram of FIG. 5.
Although it is necessary only to increase the lifting quantity of a valve body in order to eliminate such a disadvantage, a fatal limitation has occurred in that said lifting quantity cannot be increased until a thermal expansion member is enlarged when said thermal expansion member is used for a valve body-driving portion.
Also, although it is necessary to reduce the heat capacity of the thermal expansion member as far as possible in order to improve a response of the control valve in its opening and closing action, in such a case the rigidity of said thermal expansion member is reduced whereby a large flow rate of fluid cannot be controlled.